In heterogeneous networks, and in a future 5G scenario, small cell deployment benefits from coordination across all cells located in the same area. The performance improvement is achieved through spectrum reuse between small and macro cells, so that macro spectrum can be fully reused by small cells. This allows the same services to be delivered using less spectrum width. Coordination also involves cell selection and interference management, in particular at the cell edges.
A tight coordination enables advanced techniques such as:
1) Coordinated multi-point (CoMP). Signals are sent to users from multiple points
2) Coordinated link adaptation. Base stations estimate radio quality between terminals and exchange scheduling decisions
3) Coordination scheduling. Instead of each cell's scheduler operating independently, scheduling is based on the tightest possible coordination between base stations
Tight coordination is possible if the baseband processing of traffic, originated by multiple cells, is centralized in a common site. By running baseband processing in a common site, it is also possible to share computational resources, for example dedicating more computational effort to heavy loaded cells or to manage “cell edge” conditions for user located at the borders among coordinated cells.
FIG. 1 illustrates a conventional centralized network 101. The network 101 comprises two Digital Units (DU) 102, each DU 102 dedicated to serve a group 105 of a plurality of Remote Radio Units (RRU) 106. Each RRU 106 is connected with a DU 102 which reserves a given amount of its processing capability to the RRU itself. The total DU 102 capability is typically dimensioned on the peak of the traffic of the served RRU group 105, to avoid blocking conditions. In this example, each group of RRUs comprises one RRU 106a managing a macro cell, and one or more RRUs 106b managing small cells. Each DU 102 has an I/O port 110, processing circuitry 111 and a memory 112, in order to carry out baseband processing and communicate with the RRUs 102.
The RRUs 106, serving cells among which tight coordination is desired, have their baseband signals processed by the same DU 102. DUs 102 are typically designed for worst-case peak loads. However, typical workload levels are much lower than the peak loads and therefore the excess capacity is not used. For example, in heterogeneous networks in particular, RRUs managing small cells can be switched on/off according to current needs. Thus, the number/type/location of cells which require mutual coordination at DU 102 may fluctuate.
In addition, hand-over procedure is always dependent on the capacity of the target radio-base station to serve the new user. If the new user is accepted, context information has to be sent to the new cell, increasing the network overhead.
The need for coordination will become more and more demanding as the mobile network evolves towards 5G. The shortage of spectrum, the increased number of small cells and the huge number of devices moving in geographical area will require a tight coordination.